1. Field of the Invention
The present invention relates to an illumination device for supplying light having a uniform illumination distribution and more particularly to an optical system having such a device.
2. Related Background Art
Illumination devices capable of supplying light rays having a flat illumination distribution are needed for a variety of applications. Examples include in exposure devices of the type for projecting a pattern on a semiconductor wafer, thereby printing the pattern of IC or LSI; photo-chemical vapor deposition (CVD) devices of the type for using ultraviolet rays or a laser beam to cause the reaction between a silicon compound and an oxidizing gas, thereby forming a SiO.sub.2 film over the surface of a substrate; and laser annealing devices of the type for irradiating the surface of an ion injected semiconductor, thereby recovering the damages of the surface and other apparatus for the fabrication of semiconductor devices.
As disclosed, for instance, in U.S. Pat. Nos. 3,296,923 and 3,941,475, it is well known that for supplying such illuminating light rays, a pair of series lenticular matrices may be used to produce a large number of secondary light sources from a collimated light beam emitted from an elliptic mirror.
Furthermore, as disclosed in U.S. Pat. No. 4,497,015, it is also known to dispose in series a plurality of optical integrators each of which functions like the above-described series lenticular matrix, thereby further improving the illumination distribution.
Light from a plurality of secondary light sources produced by a multi-beam generator such as a pair of lenticular matrices or optical integrators as described above, may be overlapped at an object surface through the use of a condenser lens so that illumination is averaged and smoothed.
In the illumination devices of the types described above, the multi-beam generator is composed of a plurality of lens elements having the same focal length and the same magnification between the incidence surface and the object surface. It follows therefore that if the intensity distribution (more correctly the beam density distribution) of the light rays impinging on the multi-beam generator is symmetrical about the center of the beam, then a uniform illumination distribution can be obtained over the object surface. However, if the light impinging on the multi-beam generator has an asymmetric beam density distribution, it becomes impossible to cancel the inclined components (gradient) of the beam density distribution. As a result, the inclination of the illumination distribution carries over to the object surface and consequently there arises the problem that a uniform illumination cannot be obtained.
Asymmetric beam density distribution is a phenomenon which is often observed with light sources, a most noteworthy example being the excimer laser, which has been lately attracting considerable attention.